If you signed up directly with the App store you will need to reach out to them directly in order to leave Sidekick.
In order to cancel your account please follow the below steps:
Open the Settings app.
Tap your name.
Tap Subscriptions.
Tap the subscription that you want to manage.
Tap Cancel Subscription.
If you would like to obtain a refund please follow the below steps:
Go to reportaproblem.apple.com.
Sign in with your Apple ID and password.
If you see the 'Report' or 'Report a Problem' button next to the item that you want to request a refund for, click it.
Follow the instructions on the page to choose the reason why you want a refund and submit your request.
If you have an App Store Subscription we aren't able to process your cancellation request/refund. You will need to do the following in order to obtain a refund:
Head to support.apple.com/itunes
Scroll down to and click on Tell Us How We Can Help
Click on Billing & Subscriptions
Select Subscriptions and Purchases
Click on Request a refund - you can do this online or through an email to the correct team with these instructions.